This method is quick and easy, but can fail if your cabal program is out of date with respect to the GHC version you are building. Here's how to install a library against a GHC build tree:

cabal install --with-ghc=<inplace-ghc> <package>

where <inplace ghc> is the path to your inplace GHC (usually $(TOP)/inplace/bin/ghc-stage2), and <package> is the name of the package.

Points to note:

This will install the package in your home directory (e.g. somewhere under ~/.cabal/lib on a Unix system), and it will register the package in your private package database, so you'll probably want to remove and unregister it by hand when you've finished.

Plan A can fail, because sometimes Cabal changes, so you might get a message like

cabal: failed to parse output of 'ghc-pkg dump'

In that case you need to use the Cabal code that comes with the new version of GHC (ie the one in your build tree). So use Plan B.

Plan B: use the Cabal library bundled with GHC

This method is slightly more work, but it does have the advantage of not installing anything in your home directory that you have to go and remove later.

Go to a directory where you are happy to keep the newly-downloaded code.

The first step can be done manually, by downloading a zip file from Hackage, or by doing a darcs get from the appropriate repo. For example:

darcs get http://darcs.haskell.org/packages/parallel

Nevertheless, cabal unpack should work for any Hackage package, even if Cabal has changed a bit. (Because fetching and unpacking is one of Cabal's less sophisticated operations.)

It is important to compile Setup.lhs with your shiny new inplace GHC, not your installed GHC. Your inplace GHC has the most up-to-date Cabal library, and that is what you want to link Setup.lhs against.

The --global flag instructs Cabal to register the package in the database in your build tree, rather than the one in your home directory (~/.ghc/...). In fact, --global is actually unnecesary (it's the default), but just in case the default changes in the future it's a good idea to get in the habit of saying whether you want --global or --user.

The --inplace flag to register tells Cabal not to copy the compiled package, but rather to leave it right where it is, and register this location in the package database in your GHC build tree